Cable of the type to which the present Invention is directed consists of a wire core covered by a coat of insulation. The core comprises a plurality of individual strands which have been twisted together, resulting in a certain amount of clearance between adjacent conductive strands. Therefore, if the insulation is removed and the core exposed, water can be drawn into the spaces between the wires by capillary action. This can continue through a substantial length of the cable, causing discoloration and corrosion of the wire strands. Moreover, if the water reaches the opposite end of the cable, it can enter the inside of the housing to which the cable terminal is connected. Thus, not only the wire strands, but also the connector housing and the connector itself can be corroded thereby.
To solve this problem, various expedients have been proposed. For instance, in the method of Japanese Unexamined Patent Publication (Hei) 5-151848, after coating by introducing a waterproof material into the interstices between the strands, a rubber insulating separator tape, having a higher melting point than that of the waterproof material, is applied to the periphery of the wire core. The rubber and plastic insulation is then coated thereon. By covering the cable with the insulation after applying the separator tape longitudinally, the deformation that would be caused by pressure on the surface of the waterproof material can be avoided, thus assuring water-tightness and excellent appearance of insulator.
The method of the aforementioned Japanese Publication brings the exposed portion of the wire core into contact with a meltable waterproof organic insulating material. This material has a melting point of more than 50.degree. C. and a viscosity of less than 200 centipoise. As a result, a filling layer is formed between the wire core and the insulation. The end of the cable is immersed in the waterproof material and a vacuum is applied to the other end of the cable. The negative pressure thus created causes the waterproof material to enter between the wire strands, thereby sealing the cable.
However, the foregoing prior art method possesses certain disadvantages. Although the waterproof material is forced between the strands by coextrusion of the strands and the material, it is nearly impossible to fill the small clearances, because the waterproof material for the inner layer is pressed inwardly from the periphery after the cable has been stranded. In addition, since the cable is heated after application of the waterproof material, the latter is extruded as an outer layer, followed by extrusion of a separate tape to be wrapped around the periphery of the outer layer. This means that a large number of process steps are required; therefore, productivity is low, leading to higher cost. In addition, when peeling off the insulating coating to expose the wire core before crimping on a wire terminal, a problem arises because the necessity for removing the separator tape, waterproof material of the outer layer, and waterproof material of the inner layer.
According to the method of Japanese Unexamined Patent Publication (Hei) 2-10614, as shown in FIG. 6, the exposed conductive area connected to a terminal is dipped into a fluid tank containing a melted electrically-insulating organic substance. With this method, as the terminal is also coated, it is necessary to provide a further process step to remove the substance from the terminal surface.
Another known manufacturing method is that of Japanese Unexamined Patent Publication (Hei) 6-84416. In this process, it is problematical as to whether the vacuum applied to one end of the cable will find its way through the spaces between the wire strands to the other end which is immersed in the sealing agent. There is, of course, no guarantee that the interstrand clearances will be continuous from one end of the cable to the other. Moreover, even if the vacuum does reach the other end, it is likely that the sealing agent will be unable to flow all the way to the end to which the vacuum is applied. Therefore, a suitable waterproof seal throughout the length of the cable will probably not be achieved.